I actually do know quite a few dogs named
Riley - but most seem to be older pets (owned by older folk).
Another name that seemed to be prevalent in my area for dogs was
Lacey (Lacie, Lacee, etc.)
I think
Riley works well for pets because it is simple, yet travels well when you yell it. Plus, it's a bit unisex as well... and friendly.
As a side note - I know a bunch of kids around kindergarten age who are named
Riley right now. Most of them are a variation:
Ryleigh, Reilee,
Rylee, etc. Both boys and girls. I think the boom in using it for humans is more recent than pets!